Super-sized sodas, cheap junk food and huge portions at restaurants might not be what’s really feeding the U.S. obesity epidemic.

Many Americans have stopped doing any sort of physical activity.

Using data from the Centers for Disease Control’s National Health and Nutrition Examination Survey (NHANES), researchers from Stanford University’s School of Medicine showed that while caloric intake appears not to have risen, inactivity has surged.

According to the CDC, NHANES is “a program of studies designed to assess the health and nutritional status of adults and children in the United States. The survey is unique in that it combines interviews and physical examinations.”

The data revealed that between 1988 and 2010, women who reported not engaging in physical activity rose from 19 percent to 52 percent. For men, the number bounced from 11 percent to 43 percent.

During that timeframe, obesity rates increased from 25 to 35 percent among women and from 20 to 35 percent among men.

Obesity has been linked to cancer, cardiovascular disease, as well as increased mortality.

“What struck us the most was just how dramatic the change in leisure-time physical activity was,” said Dr. Uri Ladabaum, associate professor of gastroenterology and lead author of the study in a statement. “Although we cannot draw conclusions about cause and effect from our study, our findings support the notion that exercise and physical activity are important determinants of the trends in obesity.”

That so many Americans engage in no physical activity could be because lives are busier and that sedentary activities such as using computers, tablets, smartphones as well playing video games is on the rise.

“This is a part of the equation that we have not looked at before,” said Dr. Gurkirpal Singh, a co-author of the study, which will appear in the August issue of The American Journal of Medicine.

The trend is likely to continue as young Americans are seeing less physical activity.

A CDC study earlier this year that utilized NHANES data showed that half of boys between 12 and 15 years old and one third of girls had adequate levels of cardiorespiratory fitness. The percentage of those children who had adequate levels of fitness plunged from 52.4 percent in 1999 to 42.2. percent in 2012.

The Stanford researchers are quick to point out that the study does not suggest that active people can eat what they want.

Singh says researchers want to emphasize how a lack of activity will affect health.

While the study defined “ideal” exercise as “more than 150 minutes a week of moderate exercise or more than 75 minutes a week of vigorous exercise,” Singh said any exercise is better than none.

“The biggest gains from physical activity occurs from zero to something,” he said. “That’s a sharp rise. The more you do the better, but the incremental advantage [of doing more] is nowhere close what is achieved from zero to something.”

He said people shouldn’t be discouraged by the ideal amounts and that small things like taking the stairs or walking a small distance can be beneficial.

There are two caveats to the research.

First, while the study shows a correlation between obesity and a sedentary lifestyle, it was observational and therefore can “not address the possible causal link between inactivity and weight gain.”

Also, since the data about activity and calorie intake are self-reported, “participants may have been tempted to under-report how much they ate.”

In an editorial published with the research, the journal’s managing editor, Pamela Powers Hannley, called the study “a clarion call.”

In a news release she called obesity a “complex, multifaceted problem linked to a variety of societal factors,” adding that just telling patients to work out won’t be enough.

“Communities, employers and local governments to enable healthy lifestyles by ensuring that there are safe spaces to exercise that are cheap or free,” she said.